74:6 PHYSIOLOGY, 



to possess the power of excreting into the soil certain of their 

 peculiar secretions. Thus Prinsep found that Lettuces and 

 Poppies excreted a matter analogous to opium ; the LeguminoscB 

 gummy matters ; the Euphorhiacea acrid matters, &c. ; and it 

 was therefore concluded by De Candolle and others, that such 

 substances were thrown off by plants because they were injurious 

 to them. It was also believed that, while such substances were 

 injurious to the plants producing them, they were beneficial to 

 others ; and as plants could not therefore be grown for any 

 length of time in soil imp-egnated with their own injurious 

 excretions, rotation of crops was necessary. (See p. 794.) 



These experiments, when repeated by Bi'aconnot, Walser, 

 Meyen, Boussingault, &c., with every precaution, did not lead to 

 the same results, but, on the contrary, to the conclusion — that 

 the eifects observed by Macaire-Prinsep were due to his experi- 

 ments having been made without sufficient care, and that no excre- 

 tions of the peculiar substances of the plant took place unless 

 the roots of such plants were injured. While it may be proved, 

 therefore, that excretions from the roots can have no influence 

 on the rotation of crops, still it is by no means proved that roots 

 do not impart some of their contents to the soil. The evidence 

 would seem to lead to the conclusion, that roots have no power 

 of getting rid of excrementitious matters like that possessed by 

 animals, but nevertheless that they do throw off into the soil a 

 portion of their contents by a process of exosmose, which appears 

 to be an almost necessary result and accompaniment of the 

 endosmose by which absorption takes place. Carbonic acid is 

 probably parted with by roots in this way. 



Roots are frequently enlarged for the purpose of acting as 

 reservoirs of nutriment in the form of starchy, gummy, and 

 similar matters for the future support of the plant. The tubercules 

 of the dahlia {fig. 237) and orchis {figs. 235 and 236) ; and the 

 roots of the turnip {fig. 243), carrot {fig. 241), and other 

 biennials, are familiar illustrations. 



2. Of the Stem or Ascending Axis. — The offices performed 

 by the stem and its ramifications are : — 1. To form a support for 

 the leaves and other appendages of the axis which have but a 

 temporary existence, and thus enable them to be freely exposed 

 to the influences of light and atmospheric air, which are essential 

 fur the proper performance of their functions and development ; 

 2. To convey air and fluid matters upwards, downwards, and 

 inwards, to the organs of respiration, assimilation, development, 

 and secretion ; and 3. To act as a reservoir for the secretions of 

 the plant. 



Special Functions of the different Parts of the Stem. — 1. The 

 Pith. — Various functions have at different times been ascribed 

 to the pith. In the young plant, and when newly formed, the 

 cells of the pith are filled with a greenish fluid, containing gum 



